1. Field of the Invention
The present invention relates to a lead zirconate titanate-based piezoelectric film, a production method thereof, a piezoelectric element using the piezoelectric film, and a liquid discharge apparatus.
2. Description of the Related Art
An actuator including an ink jet recording head is provided with a piezoelectric body having a piezoelectric property that expands and contracts with variation in applied electric field intensity, and a piezoelectric element provided with an electrode that applies an electric field to the piezoelectric body.
In recent years, in order to meet the demand for miniaturization of an apparatus, miniaturization of an actuator in cooperation with a semiconductor process technology such as a microelectromechanical systems (MEMS) technology has proceeded. In the semiconductor process technology, high-precision processing using film formation or photolithography becomes possible. Therefore, there has been actively conducted research on thinning of a piezoelectric body in an actuator.
As a piezoelectric material having high piezoelectric properties, a lead zirconate titanate (PZT)-based perovskite oxide has been widely used due to its performance. It is known that when a PZT-based perovskite oxide piezoelectric film has a morphotropic phase boundary (MPB) composition in which Zr:Ti is near 52:48, the piezoelectric constant and the electromechanical coupling coefficient thereof become maximum, which is appropriate for actuator applications.
In JP2012-99636A, it is described that in a piezoelectric element provided with a piezoelectric thin film having a laminate of a lead titanate layer and a lead zirconate layer, which have columnar structures, the compositions of lead titanate and lead zirconate in the piezoelectric thin film are caused to be MPB compositions, thereby improving piezoelectric properties.
On the other hand, as a technique for improving piezoelectric properties using a method other than that for the formation of the MPB compositions, a PZT-based piezoelectric film is doped with various donor ions having higher valences than those of substituted ions. Since the ionic valence of Zr and Ti in B-site is 4, as donor ions that substitute for B-site elements, B-site elements having an ionic valence of 5 or higher, such as V, Nb, Ta, Sb, Mo, and W have been used.
For example, in JP1995-48172A (JP-H07-48172A), a composition for a PZT-based actuator, in which A-site of PZT is doped with Sr, Ba, and/or La and B-site is doped with Sb or Nb, resulting in a composition closer to a rhombohedron side than a MPB composition. In JP1995-48172A (JP-H07-48172A), it is described that a laminated actuator which uses a rhombohedral crystal system PZT-based composition has excellent characteristics and causes a low degree of deterioration in displacement characteristics in durable use.
Attempts on donor ion doping have been examined in thin film applications. In JP2005-209722A, it is described that in order to dope a PZT-based ferroelectric film with Nb as B-site ions at a high concentration, at least one of Si, Ge, and Sn is added as A-site ions. In JP2005-209722A, compensation ions added to the A-site are a sintering aid for obtaining a thermal equilibrium state by accelerating sintering in a thermal equilibrium process by a sol-gel method, and are necessary for suppressing an increase in crystallization temperature due to the Nb doping. However, when the sintering aid is added, piezoelectric properties deteriorate, and the effect of the addition of donor ions cannot be sufficiently exhibited.
An attempt to dope PZT with Nb at a high concentration has been reported by the inventors. In JP2008-270704A, an Nb-doped PZT film in which the effect of the addition of donor ions is significantly exhibited by controlling film formation conditions in a non-thermal-equilibrium process is described. In JP2008-270704A, the production of the Nb-doped PZT film having a MPB composition succeeded.